Coatings for gemstones

ABSTRACT

The invention provides a decorative object comprising a transparent or translucent substrate having a body and at least one surface bearing a thin film coating. The coating imparts in the substrate a body color that appears substantially constant at different angles of observation. This body color is imparted in the substrate at least in part by absorption of visible radiation that is transmitted through said coating. The coating includes a high absorption layer comprising film that is highly absorptive of visible radiation. Also provided are methods of coating gems and other decorative objects, as well as methods of heat treating coated gems and other decorative objects.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/280,009, filed Nov. 16, 2005, which is a continuation of U.S.application Ser. No. 10/288,401, filed Nov. 4, 2002, now issued as U.S.Pat. No. 6,997,014, the entire teachings of each of which areincorporated herein by reference.

FIELD OF THE INVENTION

The invention provides coatings for gemstones and other decorativeobjects. More particularly, this invention provides coatings that impartdesirable color in gemstones and other decorative objects. The inventionalso provides methods for producing (e.g., depositing) coatings of thisnature, methods of heat treating coated gemstones and other coateddecorative objects to enhance color, as well as gemstones and otherdecorative objects carrying these coatings.

BACKGROUND OF THE INVENTION

The invention relates to methods of altering the appearance ofdecorative objects, such as gemstones, by coating the decorative objectswith thin film coatings that provide color via optical absorption toimprove the appearance of the objects.

A number of processes have been developed to improve the appearance ofgemstones or to create simulated gemstones. For example, methods ofdiffusing ions into gemstones (e.g., diffusing ions of titanium and/oriron into sapphire, or diffusing ions of cobalt into topaz) have beendisclosed. These diffusion methods, however, traditionally have beenlimited to specific ions and specific substrates. Moreover, diffusionmethods typically involve extremely high temperature, which frequentlycauses breakage or damage of the gemstones. Diffusion methodscharacteristically cause the added ions to become part of the crystalsurface with no distinct boundary. In fact, diffusion methods commonlyleave a gradient of ion concentration in the substrate (e.g., in agemstone). Diffusion methods typically require long processing times,commonly more than a day. Reference is made to U.S. Pat. Nos. 2,690,630and 4,039,726.

Nuclear radiation has been used to produce color centers in gemstones,giving a body color that in some cases can be improved with heattreatment. Cyclotrons and neutron bombardment are routinely used toimpart blue color in colorless topaz. This method does not involvecoating the stone. Rather, it produces color centers throughout thestone. A disadvantage of this method is the requirement for a “coolingoff” period to allow the topaz to radioactively decay to a safe level.Traditionally, it has only been possible to obtain shades of blue withthis method. Impurities in the gemstone (and the nuclear process used)determine the particular shade of blue that is obtained. Thus, it isdifficult to obtain a consistent color on any given lot of gemstones.

Rhinestones and Carnival Glass have reflective coatings layered on oneor more surfaces of a clear substrate. The coating is usually silver orsome other highly reflective material utilized to apply a mirror coating(usually silver or aluminum) onto the back (e.g., the pavilion) of afaceted glass gem. In such a coating, substantially all of the light isreflected without passing through the coating. U.S. Pat. No. 3,039,280is representative.

Commonly-assigned U.S. Pat. No. 5,853,826, issued to Starcke et al.,discloses desirable coatings for enhancing the optical properties of adecorative object, such as a gemstone. The coatings impart in thedecorative object a desirable colored appearance, wherein the color oflight reflected from the decorative object to a viewer changes withangle of observation.

Tavelite™ is a product produced by depositing thin multiple layers on atransparent substrate to produce an interference effect. The coating isbelieved to be deposited, at least in some cases, through a process thatinvolves high temperatures. When gemstones are coated at hightemperatures, considerable breakage can occur.

U.S. Pat. No. 6,197,428, issued to Rogers, assigned to DepositionSciences, Inc., is believed to disclose the coatings and depositionmethods that are used for some of the Tavelite™ products. The Rogerspatent teaches an optical interference coating that is applied oversubstantially the entire surface of a gemstone. The coating comprisesalternating layers of materials with relatively high and low refractiveindices. The coating is said to be composed of materials that aresubstantially free of absorption of light (i.e., visible radiation). Inparticular, the optical interference coating is said to impart in thecoated gemstone perceived color that is dependent on the angle ofincidence and the relative positions of the object and the viewer.

Layered coatings on a surface of a gemstone have been provided toincrease the “fire” of the stone. These techniques involve a firstcoating of a highly refractive material, with respect to the gemstone'sindex of refraction, followed by a second coating of a different highlyrefractive material. The layers are designed so that the light reflectedat each interface of each layer causes an optical interference effect.Coatings of this nature are described, for example, in U.S. Pat. No.3,490,250.

Aqua-aura™, a product of Vision Industries, is a surface treatmentproviding a single moderately saturated color. The surface treatment isproprietary, but is believed to involve a gold-based coating that isdeposited by spraying at high temperatures. The Aqua-aura stones have ametallic sheen and a substantial dichroic appearance. For manyapplications, it is desirable to provide coated stones that do not havea dichroic appearance, as stones of this nature have a particularlynatural appearance.

Atmospheric pressure chemical vapor deposition has been used to depositfilms of titanium oxide by thermal decomposition of a titanium compound(usually TiCI₄) in air.

Colored lacquers have been painted onto the pavilion of gemstones togive the stones a colored appearance. Unfortunately, these lacquers tendto have poor durability, and have been found to come off easily.

It would be desirable to provide durable coatings that can be applied atlow temperature to gemstones and other decorative objects to impart inthe decorative objects a body color that appears substantially constantat different angles of observation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a gemstone carrying a coating on its pavilionin accordance with certain embodiments of the invention;

FIG. 2 is a schematic cross-section-view of a coating in accordance withcertain embodiments of the invention;

FIG. 3 is a schematic cross-sectional view of another coating inaccordance with certain embodiments of the invention;

FIG. 4 is a side view of a gemstone having a coating over the entiresurface of the gemstone in accordance with certain embodiments of theinvention;

FIG. 5 is a side view of a gemstone carrying a coating on its crown inaccordance with certain embodiments of the invention;

FIG. 6 is a side view of a gemstone carrying a coating on its girdle inaccordance with certain embodiments of the invention; and

FIG. 7 is a side view of a gemstone carrying a coating that has amaximum thickness adjacent a culet of the gemstone and that becomesthinner with increasing distance from the culet in accordance withcertain embodiments of the invention.

SUMMARY OF THE INVENTION

In certain embodiments, the invention provides a coated decorativeobject comprising a transparent or translucent substrate having a bodyand at least one surface bearing a sputtered coating that imparts in thesubstrate a desired body color that appears substantially constant atdifferent angles of observation. The coating comprises a high absorptionlayer of film that is highly absorptive of visible radiation such thatthe desired body color is imparted in the substrate at least in part byabsorption of visible radiation that is transmitted through the highabsorption layer.

In certain embodiments, the invention provides a method for enhancingproperties of a decorative object comprising a transparent ortranslucent substrate having a body and at least one surface. The methodcomprises coating the surface of the substrate, while maintaining thesubstrate at a low temperature of less than about 200 degrees Celsius,with a coating that imparts in the substrate a desired body color thatappears substantially constant at different angles of observation. Thecoating comprises a high absorption layer of film that is highlyabsorptive of visible radiation such that the desired body color isimparted in the substrate at least in part by absorption of visibleradiation that is transmitted through the high absorption layer.

In certain embodiments, the invention provides a method for enhancingproperties of a decorative object comprising a transparent ortranslucent substrate. The method comprises coating a surface of thesubstrate while maintaining the substrate at a low temperature of lessthan about 200 degrees Celsius, and thereafter heat treating the coatedsubstrate at an elevated temperature of greater than about 200 degreesCelsius but below that at which there occurs substantial diffusion ofmaterial from the coating into the substrate.

In certain embodiments, the invention provides a gemstone having a bodyand at least one surface bearing a sputtered coating that imparts in thegemstone a desired body color that appears substantially constant atdifferent angles of observation. The coating comprises a high absorptionlayer of film that is highly absorptive of visible radiation such thatthe desired body color is imparted in the gemstone at least in part byabsorption of visible radiation that is transmitted through the highabsorption layer.

In certain embodiments, the invention provides a gemstone having a bodywith a pavilion bearing a coating that imparts in the gemstone a desiredbody color that appears substantially constant at different angles ofobservation. The coating is born only on the pavilion of the gemstone.The pavilion of the gemstone defines a culet and the coating has athickness that is greatest adjacent the culet and becomes generallythinner with increasing distance from the culet. The coating includes ahigh absorption layer of film that is highly absorptive of visibleradiation such that the desired body color is imparted in the gemstoneat least in part by absorption of visible radiation that is transmittedthrough the high absorption layer.

In certain embodiments, the invention provides a method for enhancingproperties of a gemstone. The method comprises sputter coating a surfaceof the gemstone while maintaining the gemstone at a low temperature ofless than about 200 degrees Celsius, and thereafter heat treating thegemstone at an elevated temperature of greater than about 200 degreesCelsius but below that at which there occurs substantial diffusion ofmaterial from the coating into the gemstone.

In certain embodiments, the invention provides a method for enhancingproperties of gemstones. The method comprises providing a coatedgemstone having at least one surface bearing a coating, and heattreating the coated gemstone at an elevated temperature of greater thanabout 200 degrees Celsius but below that at which there occurssubstantial diffusion of material from the coating into the gemstone.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description is to be read with reference to thedrawings, in which like elements in different drawings have likereference numerals. The drawings, which are not necessarily to scale,depict selected embodiments and are not intended to limit the scope ofthe invention. Skilled artisans will recognize that the examplesprovided herein have many useful alternatives that fall within the scopeof the invention.

Various materials have inherent color and reflectivity/transmissivityproperties that do not lend themselves well for use as decorativeobjects. Examples include such low cost transparent gem materials ascolorless quartz (SiO₂), topaz (AL₂SiO₄F₂), and beryl (AL₂Be₂Si₆O₁₈). Toenhance the decorative properties of such gem materials, pigments anddyes (colorant) have been used to provide colors in stones having cracksinto which the colorant is made to penetrate. The colorant imparts acolor change in the base gem material. The present invention teachestechniques that apply a thin (generally less than about 50,000 Å andpreferably less than about 15,000 Å) coating 40 that does not materiallychange the dimensions, the structure, or the composition of theunderlying substrate. The coating provides improved coloration pleasingto the eye of the observer.

The coating serves as an absorber of certain light frequencies toprovide color. In certain embodiments, light entering the top (e.g., thecrown, including the table) of a gemstone passes through the absorbingcoating on the stone. This imparts color in the stone, thereby givingthe stone a pleasing appearance and increasing the stone's value. Inembodiments where the coating is applied by sputtering, the process isinexpensive and of high yield. Sputtered films provide excellent filmqualities, such as desirable mechanical and chemical durability as wellas desirable adhesion to the substrate. Coating durability isparticularly important for coated gemstones and other decorativeobjects, as these objects are typically exposed to the ambientenvironment during use. Further, sputtering allows outstanding controlover coating thickness, and sputtered coatings of highly uniformthickness can be deposited repeatedly and reproducibly. It is thuspossible to obtain an exceptional degree of color uniformity for theindividual stones, not only from a single batch, but also from differentbatches. In addition, sputtered coatings can provide thickness gradientson desired surfaces, etc., which gradients are difficult, if notimpossible, to obtain using other deposition methods. When sputtered,the present coating 40 is preferably deposited while maintaining thesubstrate at a low temperature (i.e., less than 200 degrees Celsius)such that the coating 40 has a non-splotchy, uniform appearance.Coatings of this nature are especially advantageous.

In contrast, coatings applied by spray coating at high temperatures mayhave a splotchy, non-uniform appearance. It is postulated that thisoccurs when particles being sprayed pyrolize upon impacting thesubstrate. Thus, the present coating 40 is applied at a low temperaturesuch that it 40 has a non-splotchy, uniform appearance. Further, whenthe thus coated decorative object is heat treated, as described below,the coated decorative object is given a surprisingly desirable color(e.g., a color that has a surprising hue and/or is surprisinglyintense/has a surprisingly high chroma).

Thus, the invention relates to enhancing the appearance of gemstones andother decorative objects. The substrate can be formed of materials thatare found naturally in the earth, or from synthetic materials (man-madematerials, such as are made in a lab). The methods of the inventionenhance the color and brilliance of decorative objects, such as facetedor cabochon cut stones, by the application of coatings having specificabsorptive properties. In certain embodiments, the coating is appliedonly to the back (e.g., the pavilion) of a gemstone. The effect of thecoating is to modify the intensity and color of the light reflected fromthe stone to the eye of the observer.

Thus, the invention provides a decorative object, for example, a glassobject, a cut gemstone, or a natural crystal structure, such as amineral, having at least one surface that is coated to give thedecorative object an improved appearance. While the decorative object isa gemstone in certain embodiments, the properties (e.g., color) of awide variety of transparent or translucent substrates can be altered inaccordance with the present invention.

FIG. 1 depicts an embodiment wherein the substrate 10 is a gemstone. Theillustrated gemstone 10 is a multifaceted gemstone of the well known“brilliant” cut configuration. The invention, of course, is not limitedto cut gemstones, nor is it limited to any particular cut configuration.For example, the gemstone 10 can alternatively be of the well known“step cut” or “Dutch rose cut” configurations, if so desired. Thebrilliant-cut gemstone of FIG. 1 has a crown 11, a girdle 14, and apavilion 17. The crown 11 (or “front” or “top”) of the gemstone 10defines a table 12 at its top surface and has a plurality of facets 16.The pavilion 17 (or “back” or “underside”) of the gemstone defines aculet 18 at its bottom tip and also has a plurality of facets 16′.Extending between the crown 11 and pavilion 17 of the gemstone 10 is thegirdle 14. The embodiment of FIG. 1 involves a coating 40 applied onlyto the pavilion 17 of the gemstone 10, although this is by no means arequirement. For example, the entire gemstone 10 can alternatively becoated (i.e., the coating 40 can be applied over the entire exteriorsurface area of the stone 10), as shown in FIG. 4. In another embodiment(not shown), only the crown 11 (including the table 12) of a gemstone 10is coated as shown in FIG. 5. In still another embodiment, only thegirdle 14 of a gemstone 10 is coated, as shown in FIG. 6. Manyvariations of this nature are anticipated. In any event, the body of thecoated object preferably is substantially free of diffused material fromthe coating. That is, the coating 40 on the decorative object preferablyis a discrete coating carried on the surface of the object. Thus, thecoating 40 is preferably applied, and optionally heat treated, underconditions that do not cause coating material to diffuse substantiallyinto the substrate, as described below.

FIGS. 2 and 3 depict a substrate 10 bearing a coating 40 in accordancewith certain embodiments of the invention. The total thickness of thecoating 40 is very thin. Generally, the coating thickness (allthicknesses described herein are physical thicknesses unless specifiedas being optical thicknesses) is on the order of about 100 Å to about50,000 Å, and preferably is between about 100 Å and about 15,000 Å.Since the coating is so thin, it is surprising that it 40 produces suchdesirable color by absorption. Preferably, when the coating 40 isdeposited, it 40 comprises film 20 that is highly absorptive of visibleradiation (i.e., light). For example, one particular embodiment providesan absorber layer 20 comprising (e.g., consisting essentially of)vanadium oxide. Another embodiment provides an absorber layer 20comprising (e.g., consisting essentially of) substoichometric titania. Afurther embodiment provides an absorber layer 20 comprising (e.g.,consisting essentially of) superstoichiometric titania. In certainparticularly preferred embodiments, the coating 40 includes at least onefilm layer that comprises both a dielectric carrier material and adopant (or “colorant”) that is highly absorptive of visible radiation.In FIG. 2, the illustrated coating 40 consists of a single absorbinglayer 20. Preferably, this film layer 20 has a different composition(and a different refractive index) than the substrate 10. In theembodiment of FIG. 2, the absorbing layer 20 is deposited directly uponthe substrate 10. However, this is not the case in all embodiments ofthe invention, as described below.

The dielectric carrier material can comprise any desired dielectricmaterial. The term “dielectric” is used herein to refer to anynon-metallic (i.e., neither a pure metal nor a metal alloy) compoundthat includes any one or more metals. Included in this definition wouldbe any metal oxide, metal nitride, metal carbide, metal sulfide, metalboride, etc., and any combination thereof (e.g., an oxynitride).Further, the term “metal” should be understood to include all metals andsemi-metals (i.e., metalloids).

In certain embodiments, the transparent dielectric carrier materialcomprises an oxide, a nitride, and/or an oxynitride. For example, thecarrier material can advantageously comprise an oxide, a nitride, and/oran oxynitride of a metal selected from the group consisting of titanium,zirconium, silicon, tantalum, niobium, aluminum, tungsten, tin, cerium,and germanium. These embodiments are particularly desirable.

As noted above, the dopant preferably comprises material that is highlyabsorptive of visible radiation. For example, the highly absorptivedopant can advantageously comprise a metal selected from the groupconsisting of chromium, cobalt, cerium, vanadium, praseodymium,manganese, iron, nickel, copper, ruthenium, rhodium, silver, gold, andplatinum. Thus, the dopant may be present in the film 20 in the form ofa metal or metal alloy. Alternatively, the dopant may be present in thefilm as an oxide, nitride, boride, or another compound, which ispreferably highly absorptive of visible radiation. In one particularembodiment, the absorber layer 20 comprises (e.g., consists essentiallyof) silicon oxide, cobalt oxide, titanium oxide, and cesium oxide. Inanother embodiment, the absorber layer 20 comprises (e.g., consistsessentially of) silicon oxide and silver. In still another embodiment,the absorber layer 20 comprises (e.g., consists essentially of) titaniumoxide and vanadium oxide. In certain preferred embodiments, thedielectric carrier material is a compound comprising a first metal, thehighly absorptive dopant comprises a second metal, and these first andsecond metals are different.

The absorbing layer 20 (or the “absorber layer” or the “high absorptionlayer”) preferably has an optical thickness of less than about onequarter of a wavelength of visible radiation (i.e., light). Visibleradiation occurs in the wavelength range of between about 380 nm andabout 780 nm. Thus, the high absorption layer preferably has an opticalthickness of less than about 950 Å. In certain embodiments, the highabsorption layer has an optical thickness of between about 200 Å andabout 950 Å. As is well known in the present art, optical thickness isthe product of a film's physical thickness and its refractive index.

In certain preferred embodiments, the high absorption layer 20 isapplied directly to the substrate 10. For example, FIGS. 2 and 3 depictembodiments of this nature. It will be understood, however, that theinvention provides alternate embodiments wherein one or more films arepositioned between the substrate 10 and the high absorption layer 20. Inthe embodiment of FIG. 2, the coating 40 consists of a single film layer20 (i.e., the high absorption layer). In the embodiment of FIG. 3, thecoating 40 consists of two layers 20, 30, optionally having one or morefilms (not shown) positioned between these layers 20, 30. In certainembodiments (not shown), the coating 40 comprises a surprisingly largenumber (e.g., more than 20 layers, more than 100 layers, etc.) of filmlayers. Thus, in embodiments where the absorber layer 20 is applieddirectly to the substrate 10, it can be appreciated that one 30 or morefilms can be provided over the absorber layer 20 (which in suchembodiments may be applied directly to the substrate or over one or morefilms (not shown)). Regardless of its particular layer structure, thecoating 40 imparts in the decorative object 10 uniform body color thatis (i.e., appears) substantially constant or “solid” (i.e., does notsubstantially change in hue) at different angles of observation (i.e.,at different viewing angles/when moving the decorative object relativeto the observer or visa versa).

The coating 40 may produce some optical interference phenomenon. Thecoating 40 typically has a different refractive index than thedecorative object 10. As a consequence, some optical interference isproduced. This may impart a slight dichroic effect in the coateddecorative object. The coating 40, though, does not have a substantialdichroic appearance (i.e., it has a substantially non-dichroicappearance), and thus has a very natural and “real” appearance. Thus,one aspect of the invention provides a decorative object bearing acoating 40 that imparts (through absorption of visible radiation passingthrough the coating 40) in the decorative object a solid body color thatappears substantially constant at different angles of observation,wherein the thus coated decorative object has a substantiallynon-dichroic appearance).

As noted above, the coating 40 in certain embodiments comprises a secondfilm layer 30 deposited over the high absorption film layer 20. In theseembodiments, the coating 40 is preferably provided at an opticalthickness of less than about one quarter of a wavelength of visibleradiation (as described above).

In embodiments like that shown in FIG. 3, the second layer 30 cancomprise any of the materials described above with reference to the filmlayer 20. For example, this layer 30 can comprise any of a variety oftransparent, translucent, or opaque materials. Examples includes metals,metal oxides, nitrides, sulfides, and transparent carbon. Titanium,aluminum, boron, carbon, zirconium, hafnium, niobium, vanadium,tungsten, chromium, and zinc are representative useful metals.Particularly preferred are titanium and titanium oxides, and, zirconiumand zirconium oxides. The coating material can be amorphous orcrystalline and can be composed of materials generally thought to beopaque but in very thin films are at least translucent. In certainembodiments, the second layer 30 comprises a desirable mechanically andchemically durable material, such as a film comprising titanium (e.g.,titanium dioxide, titanium nitride, etc.) and/or silicon (e.g., silicondioxide, silicon nitride, etc.).

As noted above, certain embodiments provide a gemstone 10 having coating40 only on a pavilion 17 of the gemstone 10. Faceted gems are usuallyset in jewelry with the pavilion 17 protected by a setting (not shown).In many cases, the setting protects the coating 40 from mechanicalabrasion, which can occur when the gemstone 10 is worn as an ornament.Coating the pavilion 17 selectively is also advantageous in that itresults in a highly uniform color being imparted in the gemstone 10. Theviewing angle is typically limited by the setting to those anglesviewable through the top of the stone, resulting in a particularlynatural appearance of the stone 10 since the effect of the coating 40 isviewed through the stone 10. This gives the appearance of the entirestone 10 having the color imparted by the coating 40. Since the top ofthe gemstone 10 is uncoated in these embodiments, the “luster” or lightreflected off the outer surface of the stone remains the same as theoriginal stone since the reflection characteristics of the top of thestone are unchanged. This is especially desirable.

The coating can be applied by various methods. All of these methodsemploy low temperatures so as not to affect the gemstone or decorativeobject other than to coat its surface. For purposes of this application,low temperatures are defined as those not substantially affecting thechemical structure of the gemstone or decorative object, such as bymelting, decomposing, chemically activating it, diffusing into it, etc.The low temperature is preferably less than about 200 degrees Celsius.Thus, the substrate 10 is preferably maintained at a temperature of lessthan about 200 degrees Celsius during coating. The coating can thus beformed to have a non-splotchy, uniform appearance. Representative lowtemperature vapor-coating techniques include:

(1) Sputtering applies energy from a plasma (e.g., argon) to a cathodictarget material so as to eject energetic ions, atoms, and/or molecules,a portion of which then land upon and coat a nearby substrate. Theejected material may be produced by positive ions striking the cathodictarget to eject the target material. Radio-frequency or direct currentglow discharges also directly produce reactive ions, atoms, and/ormolecules for coating a substrate. In the present invention, thesubstrate may be a gemstone and its pavilion can be coated by ions,atoms, and/or molecules sputtered from the bombarded target material.This method is generally employed at subatmospheric pressures andpreferably at a near vacuum. In the present invention, the preferredmethod for coating a surface of a substrate is by reactive sputtering.For this technique, oxygen or other reactive gas (e.g., nitrogen) isadded to an inert gas to react with the sputtered target material. Whenapplying plural coats of material on the substrate, the same lowtemperature coating technique may be applied with a different coatingmaterial or by a different coating technique. In the situation ofreactive sputtering, a different target and/or a different reaction gasmay be used without moving the substrate being coated. The coating 40can also be deposited by co-sputtering (e.g., reactively) two targets ofdifferent material by selecting the respective materials of the targetsaccording to the material desired for the deposited film.

(2) Chemical vapor deposition (CVD) and physical vapor deposition (PVD)involve the passage of an active or reactive gas in an inert carrier gasacross the surface of the decorative object being coated. The reactiongas then decomposes or is caused to react with components in the gas orthe substrate to coat the substrate.

(3) Arc Source deposition is the use of direct current to ionize coatingmaterials for coating a substrate. At lower currents, a glow dischargeis produced and also may be used. The arc may be directly applied bymaking the substrate a workpiece anode. Alternatively, a plasma jet ofexcited gases may be applied to the surface of the substrate or gemstoneto coat it. For such a coating method, the atmosphere is carefullycontrolled and usually involves subatmospheric pressure. The gasinjected around the arc to be converted into a plasma may be inert,neutral, oxidizing, or reducing, depending on the particular coatingdesired on the substrate. In evaporation, two or more sources ofparticles are aimed at a heated substrate, which in a preferredembodiment is the pavilion of the gemstone. In ultrahigh vacuums, amolecular beam epitaxy apparatus may form a single crystal coating layeron the substrate.

(4) Low pressure chemical vapor deposition (LPCVD) involves the placingof the substrate in a vacuum chamber along with the coating material.The coating material is heated, typically by being placed in a heatedvessel within the vacuum chamber. Under low pressure, the chemical vaporis evaporated and deposited as a thin film layer on the substrate.

In each of these vapor-coating techniques, the thickness of the coatingcan be changed easily by modifying certain deposition conditions. Forexample, when the layer is applied by sputtering, the duration, power,deposition atmosphere, and material being sputtered determine thethickness of the deposited film. When coating the substrate bysputtering, the treatment time will vary depending on the particularapparatus, but generally ranges from about 5 minutes to about 30minutes. A particularly thin film can even be applied in a matter ofseconds.

Adjusting the thickness of the coating directly affects the color andother optical properties of the gemstone or other decorative object.Thus, the method can be used to apply different gem thicknesses todifferent parts of the gemstone yielding different hues, or differentshades of the same hue, within the same coated gemstone.

In one method, a plurality of gemstones are provided. The stones arepositioned in a sputtering chamber adapted to apply the desired coating40. In one embodiment of this method, the stones are positioned withinthe chamber in a configuration wherein the pavilion of each stone isoriented toward the cathode(s)/target(s) in the chamber. Thecathode(s)/target(s) is then energized, such that material from thetarget(s) is sputtered onto the pavilions of the gemstones. Bysputtering the target(s) while maintaining the gemstones in thisconfiguration, a particularly desirable sputtered coating can bedeposited upon the gemstones. In particular, this method has been foundto yield a coating on the pavilion of each gemstone that has itsgreatest (i.e., maximum) thickness adjacent the culet and becomesthinner with increasing distance from the culet, as shown in FIG. 7.This deposition method, and the resulting coating, is particularlyadvantageous since stronger light (light striking the top of the stoneat angles near normal to the top surface of the stone) tends to passthrough the thicker coating area (adjacent the culet), while weakerlight (light striking the top of the stone at angles further away fromnormal to the top surface of the stone) tends to pass through thethinner coating areas (further from the culet). This improves thecoloration of the gemstone, due to the greater path length of stronglight through the absorptive coating 40.

The invention also provides methods of heat treating coated gemstonesand other coated decorative objects. In some embodiments of this nature,the method comprises providing a decorative object having at least onesurface bearing a thin film coating. The method further comprises heattreating the coated object to improve the color and/or other propertiesof the object. In these embodiments, the thin film coating can be of avariety of different types. For example, the heat-treated coating can beone of the coatings described herein (e.g., a sputtered coating,optionally having a thickness of less than 50,000 Å and preferably lessthan 15,000 Å). However, the invention extends to performing the presentmethods of heat treating a coated decorative object regardless of theparticular type of coating that is born on the decorative object.

The coated decorative object can be heat treated to enhance itsproperties (e.g., color, durability, etc.) using essentially any ovenadapted to reach the desired heat-treatment temperature. Preferably, theoven is adapted to reach and maintain elevated temperatures of at least(e.g., greater than) about 200 degrees Celsius. More preferably, theoven is adapted to reach temperatures of between about 300 and about 600degrees Celsius (e.g., at least about 400-450 degrees Celsius). In manycases, the oven is capable of reaching and maintaining higher elevatedtemperatures (e.g., at least up to about 700, 900, or 1150 degreesCelsius). Thus, it can be appreciated that the method may comprisepositioning the coated object in an oven, and operating the oven so asto subject the coated object to a desired heat-treatment process.

The coated object can be subjected to a variety of heat-treatmentprocesses. Preferably, the coated object is subjected to aheat-treatment process wherein the maximum temperature is less than thatat which there occurs substantial diffusion of material from the coatinginto the decorative object. Thus, the present heat treatment preferablydoes not cause coating material to diffuse substantially into thedecorative object. Surprisingly, though, the described heat treatmentcauses a great increase, or a great change, in the color of the coatedobject. For example, a sputtered film may not impart substantial color,or the desired color, in a gemstone or other decorative object prior tobeing heat treated, whereas following the described heat treatment thecoated decorative object exhibits a great increase, or a great change,in color. It is surmised that the heat treatment advantageously causesthe coating material to crystallize (and/or causes existing crystals toexhibit further growth), and thereby improves the intensity of the color(i.e., increases the chroma) and/or reaches a particular hue.

Preferably, the coated object is exposed to an elevated temperature ofat least about 200 degrees Celsius, more preferably between about300-600 degrees Celsius, and optimally between about 400-450 degreesCelsius. Preferably, the heat treatment (e.g., which typically beginswith the coated object at room temperature) involves exposing the coatedobject to an elevated temperature, and raising this temperature to adesired maximum temperature (which is preferably at least about 200degrees Celsius, more preferably between about 300-600 degrees Celsius,and optimally between about 400-450 degrees Celsius) in a period ofbetween about 1-8 hours, and then decreasing this temperature (e.g.,typically back down to room temperature) in a period of between about1-8 hours. In one particular method, there is provided a gem (e.g.,topaz, quartz, etc.) bearing a coating of titanium oxide and vanadiumoxide (e.g., a 50-50 mixture). The thus coated gem is heat treated to(e.g., in an oven adapted to reach) a temperature of about 450 degreesCelsius in about 1-8 hours. The heat is then ramped back down to allowthe coated gem to cool to room temperature over about 1-8 hours. Giventhe relatively slow heat-up time, the coated gem is heat treated withoutbreakage. By contrast, an extremely fast heat-up time (e.g., heating acoated gem to such temperature using a torch, as may be done in about 30seconds) is preferably avoided so as to prevent unacceptable gembreakage.

The coated decorative object desirably is subjected to a heat-treatmentprocess wherein the maximum temperature does not exceed about 1150degrees Celsius, preferably does not exceed about 900 degrees Celsius,more preferably does not exceed about 700 degrees Celsius, and perhapsoptimally is between about 300 and about 600 degrees Celsius (e.g., 400degrees Celsius or 450 degrees Celsius).

Some of the heat-treatment methods described above comprise coating atleast one surface of the decorative object before heat treating thecoated object. In these embodiments, the decorative object can be coatedusing a variety of coating methods. In certain embodiments of thisnature, the method comprises vapor coating the decorative object beforeheat treating the thus coated object. For example, the method maycomprise sputter coating the decorative object (e.g., to a coating 40thickness of less than 50,000 Å, preferably less than 15,000 Å) beforeheat treating the coated object. As noted above, the decorative objectis preferably coated while being maintained at a low temperature of lessthan about 200 degrees Celsius.

In certain embodiments, the decorative object is a gemstone. Thus, themethod may comprise providing a gemstone having at least one surfacecarrying a thin film coating. The method comprises heating such a coatedgemstone to an elevated temperature of at least (e.g., greater than)about 200 degrees Celsius. Preferably, the coated stone is heated to amaximum temperature below that at which there occurs substantialdiffusion of material from the coating into the gemstone. In one suchmethod, the coated gemstone is heated to a maximum temperature notexceeding about 1150 degrees Celsius, preferably not exceeding about 900degrees Celsius, more preferably not exceeding about 700 degreesCelsius, and perhaps optimally being between about 300 and about 600degrees Celsius (e.g., about 400 degrees Celsius or about 450 degreesCelsius). In some cases, the coated stone is exposed to temperaturesexceeding these ranges, but not for so long as to cause substantialdiffusion of coating material into the gemstone.

EXAMPLE 1

Faceted white topaz gemstones were cleaned with a 5% solution of Dawndishwashing detergent in an ultrasonic cleaner for 5 minutes and rinsedwith deionized water. The parts were dried and loaded into a sputteringmachine with titanium and vanadium targets. Argon and oxygen gas wereintroduced into the chamber and the topaz gemstones were coated on thepavilion by reactive co-sputtering. The parts were rotated alternatelyunder the titanium and vanadium targets to build up many (more than 20alternating layers of titania and vanadium oxide). The argon pressurewas 10 millitorr and the oxygen pressure was adjusted to maintain thetitanium target sputtering a fully oxidized mode. Approximately 25% byvolume of oxygen was used compared to the argon. (Insufficient oxygengives an opaque metallic deposit). A film of approximately 50% Titaniaand 50% vanadium oxide was produced. The film thickness wasapproximately 0.3 microns thick. The film was transparent and very lightgray in color. The topaz gemstones were then heated in an oven to 450degrees Celsius over about 1 hour. After reaching 450 degrees Celsius,the oven was turned off and allowed to cool. This required about 1.5hours. The gemstone color after heat treatment was a uniform yellow bodycolor.

EXAMPLE 2

Faceted white topaz gemstones were cleaned with a 5% solution of Dawndishwashing detergent in an ultrasonic cleaner for 5 minutes and rinsedwith deionized water. The parts were dried and loaded into a sputteringmachine with silicon and cobalt targets. The gemstones were sputtered byrotating underneath the silicon and cobalt targets to deposit many (morethan 100) layers of silica and cobalt oxide. Argon and oxygen gas wereintroduced into the chamber and the topaz gemstones were coated on thepavilion by reactive sputtering. The argon pressure was 15 millitorr andthe oxygen pressure was adjusted to maintain the silicon targetsputtering a fully oxidized mode. The silicon target was powered by aperiodic voltage reversal pulsed DC power supply to minimize arcingcaused by silica buildup on the target. Approximately 20% by volume ofoxygen was used compared to the argon. (Insufficient oxygen gives a darkyellow brown color due to metallic silicon). A film of approximately 90%silica and 10% cobalt oxide was produced. The film thickness wasapproximately 2.5 microns thick. The film was transparent and colorless.The topaz gemstones were then heated in an oven to 450 degrees Celsiusover about 1 hour. After reaching 450 degrees Celsius, the oven wasturned off and allowed to cool. This required about 1.5 hours. Thegemstone color after heat treatment was a uniform light blue body color.

While preferred embodiments of the invention have been described, itshould be understood that numerous changes, adaptations, andmodifications can be made therein without departing from the spirit ofthe invention and the scope of the appended claims. All referencesmentioned in this application are incorporated by reference.

1. A gemstone having a body with a pavilion bearing a coating thatserves as an absorber of certain visible radiation frequencies toprovide color so as to impart in the gemstone a desired uniform bodycolor that does not substantially change in hue when viewed at differentangles of observation, said coating being born only on the pavilion ofthe gemstone, said coating including a high absorption layer of filmthat is highly absorptive of visible radiation such that the desireduniform body color is imparted in the gemstone.